1. Field
This invention relates to procedures and apparatus for production of magnesium metal by electrolysis employing molten baths of magnesium chloride along with other, more electrolytically stable, metallic salts.
2. State of the Art
Magnesium metal is commonly produced by electrolysis, wherein a direct current flows through a molten bath of magnesium chloride and other metallic chlorides. Other metallic salts, including halides, have been used. With the chloride baths, the products of the electrolysis are typically gaseous chlorine, released from a positive pole, or anode, and liquid metallic magnesium produced upon a negative pole, or cathode. Typically, the anode is of graphite while the cathode is of steel. The molten salts of the electrolytic bath are chosen to be more dense than liquid magnesium metal, so that it migrates upwardly, adhering to the anode facing surface of the cathode. In most magnesium producing electrolytic cells, the magnesium must be transferred from a production chamber to a separate magnesium collection chamber. In some, the magnesium is caught by an upwardly sloping, inverted trough at the top of the cathode below the surface of the electrolyte, to flow non-turbulently to the collection chamber, where its density impels it to the top of the bath. Others use weirs, over which the top layer of magnesium/electrolyte mixture spills over into the collection chamber. Magnesium production apparatus and procedures are widely disclosed in numerous embodiments in technical publications and in prior patents. The latter include U.S. Pat. Nos. 3,396,094, 4,055,474, 4,604,177, 4,514,269, 4,560,449 and others to Sivilotti, along with U.S. Pat. No. 4,334,975 to Hiroshi Ishizuka, U.S. Pat. No. 4,198,282 to Andreassen, and others. U.S. Pat. No. 4,055,474, for example, discloses an apparatus comprising a row of spaced graphite anodes with planar surfaces each with an opposing planar cathode surface within the molten bath, along with-troughs for transfer of the magnesium.
For most efficient cell operation, the anode-cathode distances must be reduced to a practical minimum, while providing sufficient space for upward circulation of the bath impelled by the rising chlorine gas. The chlorine forms into a bubble layer which thickens upwardly along the anode, so that the anode-cathode distance is minimum at the bottom of the anode and increases upwardly to avoid constriction of electrolyte circulation. Inaccurate relative placement will result in uneven bath circulation and uneven current flow patterns, with attendant inefficiencies and potentially reduced anode life. Accurate cathode-anode placement cannot be directly measured within the molten bath, which may approach 1400.degree. F. Prior art shows no effective method of accurately positioning of the anodes and cathodes within the molten bath.
The use of inverted trough collection leads to other cathode configuration and placement problems. The trough must be installed sufficiently distant from the anode face to avoid entrapping the upwardly rising chlorine bubbles. The recombination of chlorine and magnesium in the cells fortunately tends to occur slowly, but mixing in the trough is clearly undesirable. The upper part of the cathode plate is typically angled relatively abruptly away from the anode surface. This provides clearance for the trough width while maintaining its distance from the anode face and the envelope of chlorine bubbles.
The problem of avoiding chlorine entrapment aside, the troughs must have sufficient flow capacity for non-turbulent transfer of the magnesium. Room for such troughs is also provided by the angled out configuration of the cathode plates. However, wide shallow troughs may require inefficient increase in cell dimension, although they tend to have desirably smaller outside flat surfaces on their anode side, which reduces unwanted magnesium production thereon. If the trough configuration is too wide and shallow, the cathode may necessarily be angled out so sharply that magnesium breakaway from its surface could occur. Deeper, narrower troughs may of course be used to provide the needed flow area, but such troughs tend to undesirably increase the anode adjacent area. For example, in U.S. Pat. No. 4,055,474, the upper edge of the cathode plate is shown curled toward the anode to provide the trough 46, which, for sufficient size, may require the cathode plate to depart too distantly from the anode, as put forth above. (Prior Art FIG. 10) Another trough design utilizes a ship channel 42. (Prior Art FIG. 8) However, the long vertical leg 43 of the channel provides trough depth but has a substantial flat vertical anode-facing surface. The straddled position of channel 42 reduces the required angle-out of the cathode plate 25, but the actual flow area of the channel is however severely limited by the cathode plate itself forming its outside boundary. Other trough embodiments include metal plates formed into downwardly opening channels, but share similar shortcomings, the cathode being the outside limit of the usable flow channel (Prior Art FIG. 9) U.S. Pat. No. 2,785,121 employs a deep side "U" shape, mounted in its entirety closer to the anode than any part of the vertical, planar, cathode employed.
A critical need remains for a cathode plate and associated trough design which will provide sufficient magnesium flow area and permit close placement of the trough with respect to the anode, along with practical means for accurately locating the anode and cathode structure within the molten bath of metallic salts.